Hoists of winches, cranes, gantries, and the like generally comprise at least one lifting or hoist motor, a protective controller for this motor (hereinafter referred to as the motor controller) which can be equipped with circuit breakers, relays and the like, and an overload safety device operating on the motor controller.
The overload safety device can have a rope, cable or other load pick-up with a cable-tension measuring bridge circuit, an electrical analyzer, and a mechanism for suppression of the load peaks from periodic oscillation of the load.
The cable-tension measuring bridge is connected by two conductors to a bridge voltage supply generator and has two output conductors for transmission of the cable-tension measuring bridge output signals.
The analyzer acts in response to overload on the protective controller for the hoist motors.
The cable-tension measuring bridge can be a strain gauge bridge which is mounted to measure the loading or cable tension.
In the hoist of German Patent document (open application DE-OS No. 25 16 768) the circuits are so constructed that the output signal of the measuring bridge after amplification serves as an input signal for several voltage dependent circuits of the analyzer, which can reverse an output contacts when the value of the voltage at its input and therefore the size of the load exceeds a fixed value upon the cable of the hoist motors.
A load table is provided. The output terminals of the circuit act on the protective controller so that the hoist operation on exceeding of a definite value of the load is halted. On the load table the load appears, for example in a digital representation.
The apparatus is provided with means for compensation of the dynamic load peaks which can be a cross bar for the cable with two side plates lying opposite one another and to which the strain gauges associated with the tension measuring bridge are attached.
By this arrangement an erroneous value for a load oscillating transverse to the cross bar will be compensated for or equalized, since when one cross bar side is loaded by the oscillating load, the other cross bar side is simultaneously unloaded. The system therefore responds to the mean value of the load.
This approach is only limitedly effective and suffers as to precision.
Moreover with a very steeply climbing load (for example, in setting the hook on the load) an impermissibly long shut-off delay can result. In the earlier constructions it is possible to calibrate the cable load pick-up and the analyzer separately and to interchange them independently of each other without joint calibration, which has proven to be advantageous for mounting and maintenance techniques. In order to achieve this interchangeability with the cable load pick up the null or zero point and sensitivity must be calibrated to a definite value.
Additionally the supply voltage for the measuring bridge in the analyzer, the amplifier and the offset voltage of the difference amplifiers as well as the voltage dependent circuits must be calibrated.
The precision and functional reliability of the entire arrangement depends in particular on the constancy and precision of this calibration and of the structural or circuit elements used. This is true especially when dynamic load peaks occur.